1. Field of the Invention
The invention is generally directed to the field of optical design of a lens surface and ophthalmic lens manufacture, and more specifically to blending disparate optical surfaces smoothly and continuously in an optical lens.
2. Description of Related Art
Contact lenses have been around for many years. Lens surfaces that were once limited to simple spherical profiles have given way to surface shapes that are now limited only by the ability to physically impart a surface topology described by any of a variety of complex mathematical expressions. Over the last ten years or so, toric surfaces have been put on contact lenses for the correction of astigmatism. Multifocal lenses have surfaces with various power zones for near, intermediate, and far distance viewing, while some contact lenses rely on Fresnel zones or diffractive effects for guiding light through the lens.
Wavefront sensors now routinely allow doctors to measure higher order aberrations of the eye with the intent of providing customized vision correction through lenses (contact types, IOLs, inlays, onlays, etc.) or refractive surgery resulting, ideally, in vision quality well beyond what has been achievable in the past. In order to correct these higher order aberrations, however, the optical surface of a contact lens, for example, will be non-rotationally symmetric. Every measured meridian will likely have a unique cross sectional profile.
A lens having a plurality of regions, each with distinct optical behavior, must incorporate zones which blend one region to the next. In cases where non-rotationally symmetric regions must be joined to other rotationally or non-rotationally symmetric regions, algorithms are required to calculate smooth and continuous blending zones.
One of the current methods employed in the manufacture of an optical surface involves describing the surface with a series of two-dimensional cross-sections. In the case of rotationally symmetric optics, one cross-section will suffice for this description. In the case of non-rotationally symmetric optics, multiple cross-sections are necessary to describe the desired three-dimensional surface. Alternatively, complicated mathematical techniques and associated computer power are required for complex surface shapes such as those of custom contact lenses, for example.
FIG. 1A shows a typical meridional cross-section 100 of a lens surface comprising two elements 102, 106 that must be blended one to another in order to allow machining of the lens surface. In this case, a simple arc 104 can be used to join the first and second segments as displayed in FIG. 1B (as is traditionally the practice). FIG. 2A shows a cross-section 200 typical of what might be found in a non-rotationally symmetric surface. FIG. 2B demonstrates that a simple arc cannot be used to make such a cross-section smooth and continuous. In this case, a more complicated, higher order algorithm is required.
U.S. Pat. No. 5,452,031 to Ducharme describes the use of piece-wise polynomials in the form of splines that are used to connect points (or knots) to define a smooth cross-sectional surface profile. Although the Ducharme patent is not expressly limited to rotationally symmetric surfaces, the practical application may be so limited. Furthermore, Ducharme's spline surfaces do not describe the optics of the lens. Roffman et al. U.S. Pat. No. 5,650,838) describes a method for programming smooth junctions between adjacent regions of a lens which have different thickness or radii of curvature. Roffman et al. relies on piece-wise linear functions or combinations of spherical and aspherical conic equations, and requires that the junction pass through a mid-point of the two disparate sections as shown in FIG. 2 of that patent. It is not clear how the Roffman et al. approach can be applied to toric lens surfaces. Barsky (U.S. Pat. No. 6,241,355) describes a method of computer-aided contact lens design and fabrication using spline-based mathematical surfaces. Barsky's high order mathematical techniques are described therein as being applicable for designing lens topology and optics.
In view of the foregoing, the inventor has recognized a need for a flexible algorithm that can be applied to virtually any two generic surfaces, and, particularly to non-rotationally symmetric surfaces, to produce a smooth and continuous blend, and for a more simplified approach than those offered by current and past techniques, requiring a minimal amount of computing time to determine the smooth and continuous blend surface, and which can be implemented in a numerically controlled machine or other optical surface processing apparatus.